Anyway, I will try to breifly explain what is the use of capacitor on tweeters.

1. Function of capacitor - it act as a high pass filter which means it will block low freq components and let the high freq components pass through. Capacitor will attenuate the low freq at 6dB/Octave.

2. How to improve the sound quality - birds sound freq around 1KHz to 16KHz. With the right value of capacitor, the low freq components < 1KHz will be filtered off to give clear birds sound.

3. What value to use? - That depend on what type of tweeter you use. For Magnetic Tweeter, the nominal impendance is around 4ohm to 8 ohm. Assume 8ohm impendance is used and the cut-off freq is 900Hz, so you need to use around 22uF capacitor.

For Piezo tweeter, theoritically no cap is needed as piezo itself is capacitance in natural. It has around 0.12uF. If you refers to its impedance chart, the value from 100ohm at 20Hz to around 25ohm at 20KHz. That means it itself will block most of the low freq components. But if you insist to use capacitor, you can use 4.7uF to 10uF value.